Pi -bonding interactions

The greater F-----F pi bonding interaction in the cis isomer is reflected in a greater... 

In extractive distillation, a third component is added to extract one of the other components. For example when cyclohexane (b.p. = 80.8 "C) is formed by hydrogenating benzene (b.p. 80.1 °C), the desired product cannot be separated by an ordinary distillation. Aniline (b.p. = 184 C) is added to form a complex with benzene, probably by a pi bond interaction. This complex boils at a much higher temperature than benzene, and the cyclohexane then can be separated by distillation. 

Table 5.7. demonstrates the crucial role of the large flat aromatic part of the modifier molecules, especially in the case of the anthracenyl-group ee = 83%). But in the case of the tripticenyl moiety the enantioselectivity falls almost to zero, probably due to steric hindrances and the difficulties of the pi-bonding interactions with the surface Pt atoms. However, according to Balandin and Klabunovskii (see review by Skvarchenko et al. ) tripticene derivatives, despite their steric molecular hindrances, could be hydrogenated... 

Pi bonding interaction between four ligand orbitals... 

TABLE 16.13 Pi bonding interactions between the iigands in the positions defined by Figure 16.29 and each of the c/-orbitais (in units of e J. 

CT 1,3,5 cyclohexatriene, in which there is no pi bond interaction and no sigma bonds holding the fragments together. This hypothetical species is modelled by three acetylenes, benzene. 

Electron correlation is often very important as well. The presence of multiple bonding interactions, such as pi back bonding, makes coordination compounds more sensitive to correlation than organic compounds. In some cases, the HF wave function does not provide even a qualitatively correct description of the compound. If the weight of the reference determinant in a single-reference CISD calculation is less than about 0.9, then the HF wave function is not qualitatively correct. In such cases, multiple-determinant, MSCSF, CASPT2, or MRCI calculations tend to be the most efficient methods. The alternative is... 

Geometrical Isomerism. Rotation about a carbon-carbon double bond is restricted because of interaction between the p orbitals which make up the pi bond. Isomerism due to such restricted rotation about a bond is known as geometric isomerism. Parallel overlap of the p orbitals of each carbon atom of the double bond forms the molecular orbital of the pi bond. The relatively large barrier to rotation about the pi bond is estimated to be nearly 63 kcal mol (263 kJ mol-i). 

The rationale behind this choice of bond integrals is that the radical stabilizing alpha effect of such radicals are explained not by the usual "resonance form" arguments, but by invoking frontier orbital interactions between the singly occupied molecular orbital of the localized carbon radical and the highest occupied molecular orbital (the non-bonding electrons atomic orbital) of the heteroatom (6). For free radicals the result of the SOMO-HOMO interaction Ts a net "one-half" pi bond (a pi bond plus a one-half... 

Based on the fact that pi-acids interact with the trinuclear gold] I) pi-bases, TR(carb) and TR(bzim), the trinuclear 3,5-diphenylpyrazolate silver(I) complex was reacted with each. Mixing [Au3(carb)3] or [Au3(bzim)3] with [Ag3(p,-3,5-Ph2pz)3] in CH2CI2 in stoichiometric ratios of 1 2 and 2 1 produced the mixed metal/mixed ligand complexes in the same gold-silver ratios. The crystalline products were not the expected acid-base adducts. It is suspected that the lability of the M-N bond (M=Au, Ag) in these complexes results in the subsequent cleavage of the cyclic complexes to produce the products statistically expected from the stoichiometry of materials used [74]. As a result of the lability of Au-N and Ag-N bonds, and the stability of... 

Fig. 7. Stereoview of the crystal structure of water solvated host 5 (folded conformation). The structure is held together by host-host and host-solvent hydrogen bonding interactions. Within the solvation layer there are chains of circular H-bonds between the molecules of water (crystal data a - 8.227, b = 8.964, c - 16.945 A, a = 89.64, / = 97.51, y = 114.28°, space group Pi taken from Ref,3S>)...

Fig. 8. Stereoscopic illustration of the inclusion compound of host 5 (folded conformation) with acetic acid and 2 mol of water. Host-host and host-water hydrogen bonding interactions stabilize the structure. The solvation layers consist of cyclic carboxy dimers of acetic acid surrounded by water species (crystal data a = 7.857, b = 11.379,c = 13.831 A,a = 92.50,/i = 101.21, y = 101.12°, space group Pi taken from Ref. 351)...

The following question remains Why are the sigma-type interactions weaker than the pi-type interactions in this case This is apparently a geometrical effect, enforced by the relative sizes of M and X valence and core orbitals in the ionic bonding environment, and illustrated in the NAO diagrams of Fig. 2.18. We may... 

Dative bonding can also occur in pi-type interactions, particularly involving the BH2 group. A good example is H2B=NH2, isoelectronic to ethylene, which is well described by a double-bonded NBO Lewis structure with dative 7Tb n bond ... 

The idealized Lewis structures are modibed in each case by donor-acceptor interactions involving the filled (Lewis) 7ta and unfilled (non-Lewis) 7ta NBOs of the formal pi bond, 7ra 7tb and7tb 7ta, nb 7ta, or7ta- nb for the three prototypes shown in (3.101). 

In the limit that nearest-neighbor contributions dominate, such a one-center nb automatically leads to an approximate 21/2-fold increase in overlap and two-fold increase in interaction energy, compared with a two-center 7tb donor. A corresponding enhancement results when the pi-acceptor is reduced from two-center (7tb ) to one-center (nb ) form, i.e., a valence p-type vacancy. Unlike the intrinsically bidirectional character of conjugation between two pi bonds (7ta->-7tb, 7tb 7ta ), the interactions of a pi bond with a nonbonding center are intrinsically mono-directional and lead to uncompensated transfer of pi charge from one moiety to the other. 

---